Electroless plating is a process in which a metal, e.g., copper, is plated on a prepared surface in a non-electrolytic chemical process. In an electroless copper plating process, a bath is provided which includes: a cupric salt, e.g., cupric sulfate; a hydroxyl-containing compound, e.g., NaOH; a chelating ligand for cupric ion, e.g., sodium ethylenediaminetetraacetate (sodium EDTA) or 1, 1', 1", 1"'-(ethylenedinitrilo)tetra-2-propanol (Quadrol); and a reducing agent, such as formaldehyde. The surface to be plated is treated with a catalyst, whereupon exposure of the treated surface to the bath results in reduction of cupric ion to the zero valence state and deposition of metallic copper on the surface.
One typical prior art bath initially contains about 0.04 molar cupric sulfate, about 0.12 molar chelating agent, about 0.2 molar formaldehyde and about 0.3 molar sodium hydroxide. The pH is typically in the range of about 12-12.5, whereat copper plating in the presence of formaldehyde is near maximal efficiency, yet, the pH is not so high as to destabilize the bath. The components of the bath are initially provided in concentrations intended to optimize efficiency of plating, and it is attempted in the process of plating and electrodialysis to always maintain optimal concentrations in the bath, although this is probably unattainable.
U.S. Pat. No. 4,549,946 issued Oct. 29, 1985 to Horn, the teachings of which are incorporated herein by reference, describes in substantial detail several approaches to build-up of waste in a copper plating bath and replenishment of plating chemicals, beginning with a simple, but inefficient, bail-out system in which a portion of partially spent bath is discarded and appropriate chemical components are added to replenish the bath and going on to discuss various proposed methods of regenerating plating baths which involve less discard of chemicals.
A typical electroless plating bath is described in U.S. Pat. No. 4,289,597 issued Sept. 15, 1981 to Grenda, which bath contains cupric sulfate, NaOH, a chelating ligand (L) and formaldehyde. The cupric sulfate is the copper source; formaldehyde is the reducing agent; the chelating ligand maintains cupric ion in solution; and the sodium hydroxide provides hydroxyl ions which are consumed during copper reduction and also provides a high pH, i.e., in the range of about 11.5-13, whereat cupric reduction by formaldehyde is at near maximal efficiency. Because formaldehyde and cupric ions are consumed during cupric ion reduction, these chemical species must be replenished by addition to the bath. Excess sulfate ion, which builds up due to cupric sulfate replenishment, and formate ion, which is the oxidation product of formaldehyde, must be removed, or else the bath will show a progressive deterioration in its plating properties. Also, hydroxyl ion is consumed during cupric ion reduction and must be replenished. In a three-compartment electrodialysis cell described in the Grenda U.S. Pat. No. 4,289,597, the teachings of which are incorporated herein by reference, hydroxyl ions are gendrated in situ and supplied to the bath while excess sulfate ion and formate ion are removed from the; bath by electrodialysis.
The electrodialysis cell described in the Grenda patent comprises three compartments defined by two anionic permselective membranes, including (1) a cathode compartment containing an aqueous sodium hydroxide solution, (2) a center compartment containing partially spent copper plating bath and (3) an anode compartment containing waste chemicals, such as sulfuric acid. Copper bath, containing chelated cupric ions, formate ions, sulfate ions, and sodium ions, is continually recirculated between an electroless copper plating chamber and the center compartment of the electrodialysis cell. The electrodialysis cell replenishes the bath with hydroxyl ions and removes formate and sulfate ions from the bath.
The bath also contains carbonate ions which form from absorbed carbon dioxide. Carbonate ions are also removed by electrodialysis, and a "steady state" of carbonate ion concentration is generally achieved. For purposes of simplicity of discussion herein, carbonate ions are largely ignored.
The principle of the three-chamber dialysis cell is that hydroxyl ions are continuously generated at the cathode, and the anionic permselective membrane permits a substantially one-way flow of anions from the cathode compartment to the center compartment and from the center compartment to the anode compartment; hydroxyl ions flow from the cathode compartment to the center compartment, and hydroxyl, carbonate, sulfate and formate ions flow from the center compartment to the anode compartment. Cations, such as Na.sup.+, are retained in the respective compartments by the anion permselective membranes. Attendant the generation of hydroxyl ions in the cathode compartment is the evolution of hydrogen. In the anode compartment, hydrogen ion is generated, oxygen is evolved, and some formate is oxidized to carbon dioxide, which is also evolved. Sulfate ions and formate ions remain in the anode compartment in the form of sulfuric acid and formic acid which are considered waste and must be removed. In the center compartment, there is a net replacement of sulfate and formate ions by the hydroxyl ions which are generated, in situ, in the cathode compartment. Accordingly, except for incidental loss, there is no need to replenish the bath with sodium hydroxide. The bath must be replenished by addition of copper sulfate and formaldehyde, but the excess sulfate and formate ions which build up during the plating process are continuously removed in the electrodialysis cell.
More sophisticated examples of electrodialysis cells of this type are described in above-referenced U.S. Pat. Nos. 4,549,946 and in 4,600,493 issued July 15, 1986 to Korngold, the teachings of which are incorporated herein by reference. The present invention is directed to the more efficient use of such electrodialysis cells.
The syntheses of OH.sup.- and H.sup.+ ions (electrolysis of water) are essentially 100% electrically efficient. The, point of issue is the net efficiency of OH.sup.- regeneration to the plating bath. This is defined as that proportion of the total OH.sup.- synthesis which migrates to and then remains in the center or electroless copper bath compartment. It is appreciated that 100% of the total OH.sup.- synthesis is always transferred across the anion permselective membrane from the catholyte to the electroless copper bath (center) compartment. Because cations are not simultaneously transferred, in order to preserve electrical charge-balance, a correspondingly equal flux of anions must transfer from the electroless copper bath compartment thru the second anion permselective membrane to the anolyte. An equimolar amount of H.sup.+ ion is simultaneously synthesized in the anode compartment relative to the OH.sup.- ion synthesis in the cathode compartment.
The anions able to transfer to the anolyte are SO.sub.4.sup.=, HCO.sub.2.sup.-, CO.sub.3.sup.=, and OH.sup.-. If a large proportion of OH.sup.- ions transfer to the anolyte, the net efficiency of OH.sup.- regeneration is low. It is the purpose of this invention to retard the transfer of OH.sup.- ions from the bath relative to other anions and thus increase the net OH.sup.- efficiency of OH.sup.- regeneration.